nLab Strøm's theorem

Statement

Strøm’s theorem

Let $(X,A)$ and $(Y,B)$ be NDR-pairs. Their product in the category of topological pairs

$$(X\times Y,X\times B\cup A\times Y)$$

is also an NDR-pair.

References

• Arne Strøm, Note on cofibrations, Math. Scand. 19 (1966) 11-14 (jstor:24490229, dml:165952, MR0211403)

• Arne Strøm, Note on cofibrations II, Math. Scand. 22 (1968) 130–142 (1969) (jstor:24489730, dml:166037, MR0243525)